The present invention relates to a silver halide photographic emulsion, a silver halide photographic material and an image forming method, which exhibit enhanced sensitivity, minimized fog density and superior contrast, improved storage stability and pressure resistance, resulting in stable high quality prints, and leading to superior image quality and enhanced print reproducibility specifically when subjected to digital exposure at a relatively high intensity for a short time.
Recent rapid directivity to digitization has led to increased opportunities of subjecting silver halide photographic materials to digital exposure. Along with such a trend, photographic color paper as a photographic material for color prints is desired with respect to suitability for exposure at a relatively high intensity for an extremely short time at the level of milli-seconds to nano-seconds and aptitude for scanning exposure.
There have been employed silver chloride emulsions or high chloride silver halide emulsions in color paper to achieve rapid processability. Further, it is commonly known that doping iridium compounds is effective to improve reciprocity law failure characteristics as a matter of properties of silver halide emulsions. There are disclosed silver high chloride halide emulsion grains having a high bromide region in the vicinity of the corners of the grains, as described in JP-A No. 64-26837 (hereinafter, the term JP-A refers to Japanese Patent Application publication; silver high chloride halide emulsion grains in which a bromide-localized region is selectively doped with an iridium compound, thereby leading to superior latent image stability and reciprocity law failure characteristics, as described in JP-A No. 1-105940. There is also disclosed a method of forming a bromide-localized region by using fine silver bromide grains doped with an iridium compound, as described in U.S. Pat. No. 5,627,020. However, neither of the foregoing methods was sufficient for improving latent image stability in the initial stage after exposure.
In a digital exposure system of the recent subject, it was proved that sufficient practical qualities were not achieved only by known techniques for improving latent image stability, in exposure suitability at a high intensity for an extremely short time. Techniques adaptable to such a digital exposure system include, for examples, chemical sensitization and spectral sensitization suitable for formation of a bromide-localized phase, as described in U.S. Pat. No. 5,691,119; and the use of a silver iodochloride emulsion, as described in European Patent Nos. 750,222 and 772,079.
However, it was proved by studies of the inventors of this application that the foregoing techniques for improving aptitude for digital exposure was not only insufficient for improving latent image stability but also resulted in marked deteriorated pressure resistance and pre-exposure storage stability of photographic materials. It is desired to immediately solve this matter.
JP-A No. 2001-188311 discloses a method for improving reciprocity law failure and coating solution stability, in which silver halide grains contain a bromide-rich or iodide-rich phase in the vicinity of the grain surface and introduction of such a rich phase is separated into two occasions, before and after addition of mercapto compounds. However, it was proved that using only this method was insufficient for improving storage stability of silver halide emulsions.
JP-A Nos. 6-19024 and 6-19026 disclose that adding non-labile di-chalcogen compounds before or during formation of silver halide grains, or during spectral/chemical sensitization of a silver halide emulsion improves fresh fog or aging fog. Further, JP-A No. 6-19037 discloses adding such compounds in the form of a solid particle dispersion and JP-A No. 6-35147 discloses that a silver chloride emulsion containing a diaminodisulfide compound and a sulfonate compound at a weight ratio of 1:1 to 1:20 improves storage stability and performance variation caused by temperature fluctuation at the time of exposure on color photographic materials.
JP-A No. 6-202265 discloses that adding a specific disulfide compound and a specific sulfinate or seleninate compound before or during spectral/chemical sensitization results in reduced fogging and enhanced sensitivity. There is also disclosed a photographic element comprising a silver chloride emulsion including a water-soluble disulfide compound, leading to reduced fogging, reduced variation in fog density and sensitivity after pre-exposure storage and reduced variation in sensitivity due to temperature fluctuation at the time of exposure, as described in JP-A No. 7-72580.
However, neither of the foregoing methods was sufficient in description and effectiveness with respect to latent image stability, storage stability and pressure resistance, specifically, latent image stability, storage stability and pressure resistance of emulsions exposed at a high intensity.
JP-A Nos. 2002-182326 and 2002-162707 discloses a photographic material comprising a silver high chloride emulsion and a mercapto compound, in embodiments of which the mercapto compound is added to a coating solution or after chemical sensitization; JP-A No. 8-234354 discloses a photographic element comprising a thiosulfonate, a sulfonate and silver iodochloride grains. However, neither of them is characterized in a method of using these compounds and improvements in sensitivity, fogging, latent image, reciprocity failure, storage stability and pressure resistance were limitative ones.
It is an object of the present invention to provide a silver halide emulsion, a silver halide photographic material and an image forming method, which exhibit enhanced sensitivity, minimized fog density and superior contrast, improved storage stability and pressure resistance, resulting in stable high quality prints, and leading to superior image quality and enhanced print reproducibility specifically when subjected to digital exposure at a relatively high intensity for a short time.
As a result of extensive study by the inventors of this application in light of the foregoing problems, it was found that the foregoing object of the invention could be achieved by adding a compound relating to this invention in such a manner as defined in this invention after formation of host grains and in the stage of chemical sensitization.
Although the compound relating to this invention has been widely used as a sensitizer, antifogging agent or development inhibitor in the process of preparing silver halide emulsions, an achievement of the object of this invention is not expected from the prior art described above and surprising.
In one aspect the present invention is direct to a silver halide emulsion comprising silver halide grains containing at least 90 mol % chloride, 0.02 to 5.0 mol % bromide and 0 to 2.0 mol % iodide, wherein the silver halide emulsion is prepared by a process comprising the steps of (i) forming a silver halide grain emulsion, (ii) subjecting the silver halide emulsion to desalting to remove soluble salts and (iii) subjecting the desalted silver halide emulsion to chemical sensitization by adding a chemical sensitizer, wherein in the step (iii), at least one compound represented by the following formula (1) to (4) is added before adding a chemical sensitizer and at least one compound represented by the following formula (1) to (4) is further added after adding a chemical sensitizer: 
wherein R, R1, R2, R3, and R4 are each an aliphatic group, an aromatic group or a heterocyclic group; M is a cation; L is a divalent linkage group; and m is 0 or 1; 
wherein R11 and R12 are each an aliphatic group, an aromatic group or a heterocyclic group, or R11 and R12 combine with each other to form a ring; m1 is an integer of 2 to 6;
Another aspect of the invention is a silver halide photographic material comprising on a support at least one image forming layer, wherein the image forming layer comprises a silver halide emulsion as claimed in claim 1.
Another aspect the invention is directed to a method of preparing the above-described silver halide emulsion comprising the steps of (i) forming a silver halide grain emulsion by mixing a silver salt and a halide salt to form silver halide host grains, (ii) subjecting the silver halide grain emulsion to desalting to remove soluble salts and (iii) subjecting the desalted silver halide emulsion to chemical sensitization by adding a chemical sensitizer, wherein in the step (iii), at least one compound represented by the foregoing formula (1) to (4) is added before adding a chemical sensitizer and at least one compound represented by the formula (1) to (4) is further added after adding a chemical sensitizer.
Further in another aspect the invention is directed to an image forming method comprising the steps of (i) subjecting the above-described photographic material to scanning exposure and (ii) subjecting the exposed photographic material to photographic color processing to form images.